Radial piston fluid translating devices, including pumps and motors and devices which may function interchangeably as a pump or as a motor, often have a closed case in which a plurality of cylinders are disposed on piston spokes that extend radially from a rotor. The cylinders orbit within an eccentrically positioned race that forces radial reciprocation of the cylinders as the rotor turns.
In some devices of this kind the race or adjacent structure forms an annular chamber at the cylinder orbit which tends to accumulate leakage fluid. In some cases the device may include means for deliberately admitting a small flow of fluid into the cylinder orbit chamber for cooling and lubrication purposes. During operation, centrifugal force tends to trap a rotating volume of the fluid in the cylinder orbit chamber unless drainage or scavenging means are provided to remove such fluid.
If a sizable volume of leakage or cooling fluid remains trapped in the cylinder orbit chamber during operation, serious power wastages occur from increased drag torque or resistance to cylinder motion, from turbulence and from increased frictional heating of the fluid. Increased heating in turn requires higher cooling capacity. Aeration problems are also aggravated in systems where the leakage or cooling fluid, typically oil, is recovered and eventually recirculated through the device as working fluid.
These problems can be reduced by providing scavenging means for expelling fluid from the cylinder orbit chamber. While this is a relatively simple matter in some pump or motor configurations, complications are encountered in many others, most notably in devices designed to operate with a high working fluid pressure or at high rotational speeds or under both conditions.
One complication arises from the centrifugal force effect which acts to hold fluid in the annular chamber formed by the race or associated structure. Simple gravity drain passages are thus ineffective. In addition, the race and associated elements which define the cylinder orbit chamber in some devices are themselves rotatable and, to provide for displacement changes, are also translatable. Thus such elements do not offer fixed locations for drain passages.
At first consideration, it might appear that scavenging could be accomplished by providing a series of drain passages around the circumference of the cylinder orbit chamber to allow centrifugal force to expel fluid into a drainage collector channel. We have found that, at least in some forms of pump or motor, this does not provide a fully satisfactory scavenging action. Power losses remain high and the other adverse effects discussed above are still encountered to an unexpected degree. The prior art does not provide a truly efficient centrifugal scavenging means for radial piston fluid translating devices of the general type identified above.